This invention relates to foundry equipment and, more particularly, to continuous sand mullers for mold or core production.
In the production of sand molds or cores, it has been general foundry practice to bond the sand with clay, bentonite, sodium silicate or other such binders. Fairly recently, a wide variety of relatively quick setting resin binders have been made available to the foundry industry. These binders have relatively short working times and strip times and do not require baking for proper set. Conventional mulling equipment, due to their relatively low intensity mixing action, have not been capable of making full use of the quick setting charcteristics of the various resin binder systems. In fact, the average residence time needed to insure thorough mixing of the sand, catalyst and resin has been on the order of four to eight minutes. Further, the sand mix has a tendency to precure in conventional mullers prior to deposition in the mold flask or core box. This can result in a lumpy or streaked sand mixture which does not set to maximum obtainable physical properties.
Continuous sand mullers have also recently become available to the foundry industry. These mullers generally incorporate a single, high intensity auger disposed within a trough for thoroughly mixing the sand and binder additives. With the majority of binder systems, it has been general practice to add either the resin or the catalyst to a metered stream of sand at the entry point to the continuous muller. The remaining additive is then introduced into the sand additive mixture downstream from the entry point. The sand mixture residence time within these continuous mullers has generally been on the order of twenty-five seconds. Due to the high intensity mixing action employed in these continuous mullers, substantial heat is imparted to the mixture which may result in some precuring prior to deposition in the core box or mold flask. Further, since the binder additives and the sand are all contained in a single auger or muller, should production be stopped for some reason, a quantity of sand mixture must be dumped. The additives and sand cannot be left in the muller for any length of time without setting. In order to clean such mullers, a quantity of raw sand must be passed through them. This procedure is both time consuming and wasteful of foundry sand.
It has also been proposed to premix a resin with a quantity of sand in a conventional type mulling trough and a quantity of catalyst additive and sand in another trough. Quantities of each of the premixes are then deposited in a batch-type mixer. While alleviating some of the problems experienced with mixing of resin/catalyst binders totally within a conventional muller, problems have still been experienced. For example, such machines employ a batch-type mixer and, therefore, do not have instantaneous on/off sand mix capability. Therefore, should production problems such as unavailability of core boxes or mold flasks be encountered, substantial quantities of the sand mix must be dumped and therefore wasted. Also, these machines experience material buildup problems, requiring the use of raw sand for cleaning or purging purposes after each cycle.
With the various resin binder systems available, such as the no-bake furan, phenol formaldehyde, polymer isocyanate, or oil-urethane binder systems, the strip times may be decreased to the order of 25 or 30 seconds by increasing the amount of catalyst employed in the mix. Present mullers and mixing techniques have not been capable of accommodating such reduced strip times. Production has been limited, not by the binder systems, but by the availability of machines having adequate mixing ability.
A need, therefore, exists for a continuous-type sand muller having the capability of effectively and efficiently mixing foundry sand and a resin binder system with extremely low active mixture retention times. Such a system should be essentially self-cleaning, provide instant on/off control of sand and additive flow with no sand loss or time loss and with no reactive material remaining within the muller upon shutdown.